Internal combustion engines produce emissions containing water vapor, products of incomplete combustion such as, carbon monoxide and unburned hydrocarbons, carbon dioxide, oxides of nitrogen [NOx], carbonaceous soot and other combustible particulate matter, and other particulates and gaseous constituents. Oxides of nitrogen, products of incomplete combustion, and particulates are considered atmospheric pollutants. The particulate matter may also contain condensed hazardous compounds
Such emissions produce well-known harmful effects to environmental quality and human health. For example, engine soot emissions contribute to reduced atmospheric visibility and particulate fall out, and have been found to contain carcinogenic polycyclic aromatic hydrocarbons, such as naphthalene, acenaphthylene, anthracene, and chrysene. B. S. Haynes and H. G. Wagner, Soot Formation, Progress in Energy and Combustion Science, Vol. 7, at p. 229 (1990).
Further, because of its particle size, the particulate matter from diesel exhaust represents a respiratory health hazard. The particle size distribution of particulate matter from diesel engine exhaust is typically 80% minus 10 microns, and 77% minus one micron, based on aerodynamic particle diameter.
In response to air quality regulations, vehicle manufacturers install pollution control devices in internal combustion engine exhaust systems. Traditional engine pollution control devices employ a ceramic honeycomb monolith or a packed bed of pellets having a coating of a noble metal catalyst. Such devices catalyze the reactions of carbon monoxide and unburned hydrocarbons with oxygen, typically at approximately 500.degree. F. to 800.degree. F. Other devices employ catalysts that also catalyze the reaction of oxides of nitrogen. Unfortunately, two factors render such catalytic devices unsuitable for soot-laden gases that are commonly produced by diesel engines. First, the catalytic devices are ineffective at destroying soot. Second, the soot and other particulates deposit on the monolith, thereby preventing gaseous constituents from reaching the catalytic material, or possibly deactivating or poisoning the catalyst. Further, spent catalyst also may be classified as a hazardous substance. Moreover, such devices induce a substantial back-pressure on the engine, which reduces engine efficiency. Further, sulfur that is found in diesel and gasoline fuels can poison or deactivate the catalyst.
A technically feasible method of reducing soot emissions is to pass engine exhaust gas through a ceramic filter that can periodically be replaced or regenerated. These filters, however, have only 85% removal efficiency, impose a significant back pressure on the engine, and are expensive. Filter manufacturers estimate that filter prices would drop no lower than U.S. $8,800 each, even with economies of scale because of increased production. Control of Air Pollution From New Motor Vehicles and New Motor Engines, Federal Register, Vol. 58, No. 55, Mar. 24, 1993, p. 15786 (1993). Furthermore, the engine back pressure caused by the ceramic filter adds U.S. $2,000 in annual fuel costs to a typical urban bus because of reduced engine efficiency. Id.
One type of filter trap design regenerates itself by burning some engine fuel periodically, thereby oxidizing the soot accumulated on the filter surface. Another trap design continuously regenerates with the use of a catalyst. The latter trap design has achieved reduction efficiency of between 80 and 92% for particulate matter. Focus on Industry Solutions for Exhaust Pollution Control, Automotive Engineer, October/November 1994, at p. 18. Unfortunately, regenerative trap features add even more to the filter cost.
Thus, no commercially viable method currently exists for removing soot and other particulates from engine exhaust gases. The lack of effective soot treatment methods is especially problematic for diesel engines that produce high soot emissions. Despite the difficulty in controlling such emissions, the U.S. Environmental Protection Agency ("EPA") has implemented regulations restricting particulate matter emissions from buses and other heavy duty engines. Control of Air Pollution From New Motor Vehicles and New Motor Engines, Federal Register, Vol. 58, No. 55, Mar. 24, 1993, p. 15781.
Although eliminating particulate matter from diesel engines has been an intractable problem, industrial gas cleaning techniques have been employed to collect particulate in other applications. One technique for collecting and removing particles from a gas stream is electrostatic precipitation, which uses electrostatically charged surfaces to collect charged particles. An electrostatic precipitator device ("ESP") imparts a charge on particles within a gas stream by exposing the particles to an electric field. Plates or cylinders, which have a charge opposite that of both the electric field and the particles, attract and collect the charged particles. Conventional ESPs intermittently clear collected particles from the collection surface. Conventional dry-process ESPs clear collected particles by mechanical methods, such as mechanical shock or rapping, and conventional wet-process ESPs flush the particles with a liquid. After the particles are cleared from the collection surface, the particles fall into a hopper for disposal. Conventional ESPs are limited by the temperature limits of the internal components, and flammable gas constituents entering a conventional ESP are controlled to avoid ignition by arcing within the ESP.
Another technique for collecting particles entrained within a gas stream is centrifugal separation using a cyclone. In a conventional cyclone, an inlet air stream is directed to form a vortex. Centrifugal forces push particles within the gas stream to the wall of the cyclone shell, where they lose momentum and fall out of entrainment. Because the collection efficiency of a certain particle size depends on the mass and aerodynamic diameter of the particle, cyclones have higher collection efficiency on larger, more massive particles. For example, cyclones are generally effective at removing particles of greater than about three to five microns. Neither a conventional cyclone nor a conventional electrostatic precipitator can effectively reduce the large component of the diesel exhaust particulate matter that has a diameter of less than one micron.
In addition to regulations governing particulate matter and hydrocarbon emissions, internal combustion engines are the subject of regulations limiting NOx emissions. Oil and Gas Journal, Jul. 25, 1994, p.42. The simultaneous emission limits for both particulate matter and NOx presents a unique problem because the two pollutants typically have an inverse relationship in engine exhaust. Internal combustion engines generally can be configured and tuned to produce emissions having low soot and high NOx concentrations or, alternatively, high soot and low NOx concentrations. Traditionally, engines that employ catalytic devices are adjusted to minimize soot formation because of the catalysts' inability to handle high temperatures inherent in combustion of soot. Tradeoffs also typically compromise engine efficiency. Such adjustments result in high levels of NOx emissions.
In efforts to comply with regulatory limits, diesel engines have been redesigned to reduce particulate emissions. Such redesigns include, for example, a dramatically different combustion chamber design, manufacturing the engine with tighter bore tolerances to reduce the introduction of oil into the combustion chamber, and increasing injection pressures. Magdi K. Kahair and Bruce B. Bykowski, Design and Development of Catalytic Converters for Diesels, SAE paper 921677, p. 199. Although helpful, such redesigns have been inadequate to meet present and contemplated future regulatory emission limits. Advanced common rail high pressure injection of fuel is the primary technology for reducing particulate mass.
Although not generally employed in reducing NOx emissions from internal combustion engines, various techniques exist for reducing NOx emissions from gas streams in other applications. One technique for reducing NOx emissions is selective catalytic reduction (SCR), which destroys NOx in the presence of ammonia (NH.sub.3) over a catalyst. Although selective catalytic NOx reduction is capable of high levels of NOx removal, the temperature of the exhaust must be in the range of 550.degree. F.-800.degree. F., which is typically below internal combustion engine outlet temperatures. Furthermore, the catalyst has the limitations discussed hereinabove.
Another approach for removing NOx is selective non-catalytic reduction (SNCR), which employs a chemical that selectively reacts, in the gas phase, with NOx in the presence of oxygen at a temperature greater than 1150.degree. F. Chemical NOx reduction agents used in such processes include ammonia (NH.sub.3), urea (NH.sub.2 CONH.sub.2), cyanuric acid (HNCO).sub.3, iso-cyanate, hydrazene, ammonium sulfate, atomic nitrogen, melamine, methyl amines, and bi-urates.
Additional recent regulations require automobile manufacturers to reduce emission of organic vapor from vehicle fuel tanks. Typically, fuel tank control devices have a layer of activated carbon that absorbs the vapors and prevents their escape to the atmosphere. Periodically, the control devices require regeneration or replacement with fresh adsorbent material. Unfortunately, these devices are complex and expensive. Id.
Another option for reducing fuel tank emissions is to process them through the existing catalytic device in the engine exhaust system. However, conventional catalytic devices are generally unsuitable for use with concentrated fuel tank vapor. Specifically, concentrated fuel vapor combustion may raise the monolith temperature above the catalyst's upper temperature limit, thereby thermally deactivating the catalyst.
Therefore, it is an object of the present invention to provide a system and method for reducing internal combustion engine pollutant emissions in response to regulatory emission limits. Specifically, an object of the present invention is to provide a system and method for reducing soot concentration in an engine exhaust stream that overcomes the limitations of the prior art.
It is another object of the invention to provide a system and method for reducing soot concentration, while simultaneously enabling the reduction of NOx concentration, in an engine exhaust steam.
It is yet another object of the invention to provide a system and method for reducing the fuel vapor emissions from an engine fuel storage tank.